ZX80 and ZX81 File formats
--------------------------

	kio 2012-10-03

.o / .80
--------

"o" and "80" files are the same and used when saving ZX80 programs.

A ZX80 program is stored like this on a real audio tape:

	x seconds    your voice, saying "filename" (optional)
	x seconds    video noise
	5 seconds    silence
	LEN bytes    data, loaded to address $4000, LEN = ($400A)-$4000.
	x seconds    silence / video noise

Notes:
	ZX80 files do not have filenames
	ZX80 files cannot be autostarted.
	The data is loaded to address $4000++
	The data contains the whole system area, basic program, VARS.
	Video memory is NOT included in ZX80 files.
	the last byte of a (clean) file should be $80 (the last byte of VARS)
	The system area should contain proper data.
	$400A       (2 bytes) defines the data end address (used to calculate the file length).
	$4028++     may be misused for whatever purpose.

	While loading, the data at address $400A/400B is overwritten. After this they contain 
	the real data end address of the data loaded and define when loading will stop. :-)

	Files should usually not exceed 16 kBytes.
	The memory detection procedure in both ZX80 and ZX81 stops after 16 kBytes (at $8000),
	and initializes the stack pointer at that address, even if more memory is installed.
	Thus loading files of 16k or more would destroy the stack area,
	unless a separate loader has previously moved the stack area to another location.
	However, most ZXes don't have more than 16k RAM, so bigger files won't load on most computers.

".o" and ".80" files consists of the raw data as saved by the ZX80 tape saving routine.
They can only store one program, not a whole tape with multiple programs.

	.80 and .o files:   include only the data, loaded to $4000++
	.o files:	    typically there is some garbage at the file end


.p / .81 files and .p81 files
-----------------------------

"p" and "81" files are the same and used when saving ZX81 programs.
"p81" files are like "p" and "81" files prepended with the program's name.

A ZX81 program is stored like this on a real audio tape:

	x seconds    your voice, saying "filename" (optional)
	x seconds    video noise
	5 seconds    silence
	1-127 bytes  filename (bit 7 set in last char)
	LEN bytes    data, loaded to address $4009, LEN = ($4014)-$4009.
	1 pulse      video retrace signal if display was enabled
	x seconds    silence / video noise

Notes:
	The data contains system area, basic program, video memory, VARS.
	the last byte of a (clean) file should be $80 (the last byte of VARS)

	$4014	defines the end address (used to calculate the file length)
	$4029	points to the next executed (autostarted) BASIC line
	$403B	indicates if program runs in SLOW or FAST mode (bit 6)
	$403C++	may be misused for whatever purpose,
	video memory must contain 25 HALT opcodes if the file was saved in SLOW mode.

	While loading, the data at address $4014/4015 is overwritten. After this they contain 
	the real data end address of the data loaded and define when loading will stop. :-)

	Files should usually not exceed 16 kBytes.
	The memory detection procedure in both ZX80 and ZX81 stops after 16 kBytes (at $8000),
	and initializes the stack pointer at that address, even if more memory is installed.
	Thus loading files of 16k or more would destroy the stack area,
	unless a separate loader has previously moved the stack area to another location.
	However, most ZXes don't have more than 16k RAM, so bigger files won't load on most computers.

".81" and ".p" files consist of the raw data as saved by the ZX81 tape saving routine WITHOUT the filename.
They can only store one program, not a whole tape with multiple programs.
".p81" files consist of the raw data as saved by the ZX81 tape saving routine INCLUDING the filename.
This format can store multiple programs.
	
	.p and .81:	include only the data, loaded to $4009++
	.p files:	typically there is some garbage at the file end
	.p81 files:	start with the 1..127 bytes filename, last byte ORed with $80